Why did fuzzy logic fall out of fashion?
Fuzzy logic is not down trending.  It's a common architecture selection for systems that require the representation of uncertainty in changing rules.  The domains include work-flow control, aeronautics, chemical plant engineering, automated defense of cyber-attack, building systems, and business intelligence.

In natural language processing, it's up-trending under other names.  Semantic nets are a logical structure with strengths assigned between nodes of the net based on meta rules, which is reminiscent of early fuzzy logic software.

For instance, the rule might be that a word that ends with -ly before a word associated with action may be an adverb modifying a verb in English, but it is not always the case.  The rule's application is not smooth, meaning non-probabilistic.  It is fuzzy.

There is no way to create a grammar for common speech or writing in any natural language.  Language is dynamic and as practically non-deterministic as wind vectors.  That's why grammar is a down-trending word among linguists and people working in NLP.

Cognition is expected to require fuzzy valuations to the black and white nature of early production systems developed at MIT, Cambridge, the U.S. Navy, Yale U, and other locations in Eurasia.  People develop rules in their cortex and they rise and drop in probability when decisions are made after the rule seemed to have worked or didn't.

The logic that works to prove a math theorem may require discrete inference, but people are smart when they can weigh alternatives and apply rules in unique ways to partial data and limited experience.